When drilling oil and/or gas wells, it will often be necessary to guide the drilling tool in a desired direction out of the tool's drilling direction. This is the case, for example, in connection with directional wells which may have a substantial deviation from a vertical direction.
During drilling it is common to use a drill bit connected to a drill string, which drill string and drill bit are rotated about their longitudinal, axial axes by a drive unit which may be located at the surface.
Directional control of the drilling tool can be affected by applying to the drill bit a radial force which is designed to drive the bit to drill at a certain deviation in a desired direction in relation to the centre axis of the bit. The drilling tool may also have provided therein a gyroscope, one or more accelerometers or one or more magnetometers which give feedback on the position of the drill bit in the ground being drilled in.
With regard to directional drilling, it is common general knowledge that it can be done in two ways: either by so-called “point-the-bit”, where the drill bit is “tilted” about a point in a desired drilling direction and at a desired angle relative to the centre axis of the drill bit, or by so-called “push-the-bit”, where the drill bit is pushed sideways.
An example of a “point-the-bit” solution is described in U.S. Pat. No. 6,092,610. The disadvantage of this solution is that it requires continuous rotation of the bearings for the drill bit shaft at the same speed as, and in the opposite direction to, the drill string. Another example of a “point-the-bit” solution is described in U.S. Pat. No. 6,581,699 B1. The drawback of this solution is that it requires a great deal of force in order to bend the drill string, in addition to the possibility of problems of fatigue arising due to rotation whilst the shaft is bent.
A known “push-the-bit” device is described in WO 2008/156375, where three steering bodies are used that are arranged around the drilling tool in the circumferential direction and are movable in a radial direction in order to push the drill bit in the desired direction. The three steering bodies, which are individually movable, may, however, be subject to problems during operation, for example, leakages at the seals around the movable steering bodies.
Furthermore, WO 96/31679 teaches the use of two eccentric shafts for adjustment of drilling deviation. With this solution it is not possible to achieve a good adjustment in the opposite direction of the pregnant or weighted side 20. This means that the tool taught in this document is rather inflexible in use.
A tool is also known from GB 2334601 where eccentric shafts are used for adjustment of deviation and direction of the drilling tool. This is a “point-the-bit” solution which bends the drill string by means of eccentric rings between two stabilising units.
WO 2005/099424 describes a rotary gear mechanism that is used to control deviation and direction of the drilling tool. This document teaches that eccentric shafts are used to determine the magnitude of the deviation and the direction. However, this apparatus is constructed such that it is not possible to adjust direction and deviation separately; if direction is adjusted, deflection must also be adjusted or vice versa.